The field of data storage using optical record carriers is currently an intensively researched area of technology. Many such optical record carrier formats exist including compact discs (CD), conventional digital versatile discs (DVD) and Blu-ray discs (BD). These formats are available in different types including read-only versions (CD-ROM/DVD-ROM/BD-ROM), recordable versions (CD-R/DVD-R/BD-R), re-writeable versions (CD-RW/DVD-RW/BD-R) and audio versions (CD-A). For scanning the different formats of optical record carrier it is necessary to use a radiation beam having a different wavelength. This wavelength is approximately 780 nm for scanning a CD, approximately 650 nm for scanning a DVD and approximately 405 nm for scanning a BD.
Different formats of optical disc are capable of storing different maximum quantities of data. This maximum quantity is related to the wavelength of the radiation beam which is necessary to scan the disc and a numerical aperture (NA) of the objective lens. Scanning can include reading and/or writing of data on the disc.
The data on an optical disc is stored on an information layer. The information layer of the disc is protected by a cover layer which has a predetermined thickness. Different formats of optical disc have a different thickness of the cover layer, for example the cover layer thickness of CD is approximately 1.2 mm, DVD is approximately 0.6 mm and BD is approximately 0.1 mm. When scanning an optical disc of a certain format, the radiation beam is focused to a point on the information layer. As the radiation beam passes through the cover layer of the disc a spherical aberration is introduced into the radiation beam. An amount of introduced spherical aberration depends on the thickness of the cover layer and the wavelength of the radiation beam. Prior to reaching the cover layer of the disc the radiation beam needs to already possess a certain spherical aberration such that in combination with the spherical aberration introduced by the cover layer, the radiation beam may be correctly focused on the information layer of the disc. For scanning different discs with different cover layer thicknesses, the radiation beam needs to possess a different spherical aberration prior to reaching the cover layer. This ensures correct focusing of the radiation beam on the information layer
It is desirable to have one optical device which is capable of scanning many different formats of disc, for example CD, DVD and BD. Such devices are often relatively difficult to design. This is in part because different cover layer thicknesses require a different spherical aberration of the appropriate radiation beam prior to reaching the cover layer. Such multiple disc format scanning devices often include an assembly of many different optical elements which are individually specific for the scanning of only one optical disc. This often makes such devices relatively complex and consequently bulky and expensive.
International patent application WO 03/060891 describes an optical scanning device for scanning an information layer of three different optical record carriers using, respectively, three different radiation beams. Each radiation beam has a polarisation and a different wavelength. The device comprises an objective lens having a diffractive part which comprises birefringent material. The diffractive part diffracts the radiation beams such that the beam with the shortest wavelength has an introduced phase change modulo 2π of substantially zero for the shortest wavelength. The diffractive part diffracts at least one of the other radiation beams into a positive first order.
International patent application WO 03/060892 describes an optical scanning device for scanning an information layer of three different optical record carriers using, respectively, three different radiation beams. Each radiation beam has a polarisation and a different wavelength. The device comprises an objective lens and a phase structure for compensating a wavefront aberration of one or two of the radiation beams. The phase structure comprises birefringent material and has a non-periodic stepped profile.
U.S. Pat. No. 6,687,037 describes an optical scanning device for scanning optical record carriers with radiation beams of two different wavelengths. The device comprises an objective lens and a diffractive element having a stepped profile which approximates a blazed diffraction grating. The diffractive element selects a zeroth diffraction order for the radiation beam of the shortest wavelength, and selects a first order for the other radiation beam.
International patent application WO 02/41307 describes a lens system for use in an optical scanning device. In this system, a radiation beam is used to scan an information layer of an optical record carrier. A lens of the system has both a diffractive grating and a phase structure having a non-periodic stepped profile. This lens reduces a sensitivity of the lens system to variations in the wavelength of the radiation beam and to variations of environmental temperature.